Undersampling is defined as sampling a signal at a rate that is less
than the desired Nyquist rate of two times the highest frequency in the signal to be
digitized. As you have seen, undersampling causes aliasing that upon recovery of the
signal in a DAC produces a signal that is much lower in frequency. This is generally an
undesirable effect that can be eliminated by adding a low-pass antialiasing filter in front
of the ADC to cut off signals that are more than half the sampling rate.